The high purity L-(+)-lactic acid has several industrial applications. The L-(+)-lactic acid so produced can be used in the production of dairy products, as an acidulating agent in the alimentary field, as an intermediate for the production of plasticizer agents, adhesives, pharmaceutical products, in the production of lactates, as a mordant in wool dying and so forth. Similarly, the high purity L-(+)-lactic acid prepared from a highly pure S-(−)-methyl lactate has considerable prospects of industrial development is in the field of production of biocompatible and biodegradable polymers which are useful for manufacturing bags, application films, in the field of sanitary field, medical applications and so on.
Purification of L-(+)-lactic acid; there are various manufacturing methods reported in the prior art namely, electro dialysis, extraction with base, adsorption and de-sorption, reactive distillation to prepare alkyl lactate followed by the hydrolysis of alkyl lactate to get lactic acid etc. This invention deals with the a process for the preparation of pure L-(+)-lactic acid from reasonably pure S-(−)-methyl lactate (˜95-98% purity) obtained from calcium lactate which is formed by sugar cane juice fermentation. The manufacturing method for lactic acid is known from the prior art and in general, comprises three steps. In the step 1, alkali metal lactate is reacted with concentrated or dilute sulfuric acid. The corresponding alkali metal sulfate is produced as the side product along with the dilute aqueous solution of lactic acid. In the step 2, the dilute lactic acid solution is concentrated and esterified with methanol using a catalyst. In most of the invention the resultant mixture of methyl lactate, methanol and water is hydrolyzed with acidic catalyst to get lactic acid solution in water. Various methods of manufacturing lactic acid based on the above general method have been suggested in the past as cited in the following patented literatures:
Reference may be made to the U.S. Pat. No. 1,695,449, where in the manufacture of esters of lactic acid, which was done in a batch mode by boiling of 75% lactic acid solution with alcohol along with aluminum sulfate and removing the water of reaction by constant boiling mixture using benzene. Once the water of reaction is removed, the mixture is cooled and filtered to remove aluminum sulfate, neutralized to remove un-reacted lactic acid and ester then recovered by fractional distillation. The conversion of lactic acid reported in this run is 94.5 to 96.5%. However, the yield is not reported. This can be given as a typical example. The drawbacks of the of the above-mentioned prior art are, it can be seen that although the conversion of lactic acid is higher at 94.5%, the molar ratio of alcohol to the lactic acid is claimed to be greater than or equal to 8. Such use of excess of alcohol in the process leads to higher capital investment on recovery as well as higher operating cost of recovery at the industrial scale of manufacture. Further drawback is, this method requires the reaction to be carried out at constant boiling point mixture at 64.8° C.; at this temperature rate of conversion of lactic acid to its ester is not very high (Refer Ralph A. Tropue and Kenneth A. Kobe, Industrial Engineering Chemistry 42 (5), 801-810 (1950), Ralph A. Tropue and Kenneth A. Kobe, Ibid, 42 (7), 1403-1409 (1950)). The lower reaction temperature requires higher time to attain the desired conversion and directly affects the throughput and capital investment. Similarly in order to push the reaction to forward direction the water in the lactic acid and water of reaction has to be removed azeotropically with benzene. This requires distillation and recycles of benzene, many folds to the original water in the lactic acid and water of reaction. Similarly the distillate obtained may be in single phase or in two phases. Recovery of excess alcohol and benzene becomes a complicated operation, which also affects the process cost. Overall, this calls for larger reaction time and higher capital investment.
Reference may also be made to the U.S. Pat. No. 2,029,694, wherein the method for manufacture of methyl lactate is described. Aqueous lactic acid is first dehydrated by heating to about 140° C., whereby the lactic acid gets converted to lactide and lactyl lactate. This is then reacted in a batch mode with small amount of methanol and sulfuric acid as the catalyst. Distillation of the reaction water, excess methanol and methyl lactate is carried out simultaneously at about 125° C. At the same time methanol is introduced in the reaction mixture held at 130 to 140° C., whereby the water of reaction, excess methanol and methyl lactate is distilled out in the co-current mode of the reactive distillation, from reaction mixture. The yield of methyl lactate based on lactic acid reported is around 83% and the mole ratio of methanol to lactic acid is around 3.0. The main drawback of this process is that it gives lower yield based on lactic acid despite using higher reaction temperature, which will affect the overall process economics. Further drawback is the higher reaction temperature promotes the polymerization of lactic acid and the formation of objectionable impurities like hydroxymethylfurfural or other undesired products. Some of these impurities are volatile in nature and distill along with methyl lactate. The distillate obtained by this process contains lot of water, methanol and methyl lactate and isolation of methyl lactate in pure form needs lot of additional energy. This is generally called as co-current approach of reactive distillation, where the product and bye-products are removed as the top product as a distillate, while the esterification is in progress. The product i.e. methyl lactate being higher boiler the temperature of the esterification still needs to be maintained at higher temperature to drive out the product and the bye product. While investigating this approach i.e. co-current approach for reactive distillation, we find 2-pentene-1-ol [1576-96-1] as the bye product and the impurity, along with hydroxymethylfurfural as reported in this patent i.e. U.S. Pat. No. 2,029,694. This was repeatedly observed in all the experiments where the distillation still temperature was 125° C. or more. It becomes very difficulty to remove this bye product 2-pentene-1-ol [1576-96-1] from methyl lactate. As per the available literature information, in the recovery cycle of methyl lactate, the side product 2-pentene-1-ol [1576-96-1] reacts with methyl lactate and trans-esterification occurs leaving away methanol as the bye product of the trans-esterification process. The bye product formed after trans-esterification of the methyl lactate i.e. bye product pentyl lactate is very difficult to hydrolyze to lactic acid, in the subsequent hydrolysis cycle of methyl lactate to recover pure lactic acid.
Reference further may be made to the U.S. Pat. No. 2,290,926 wherein the alkaline earth metal lactate preferably sodium lactate is heated along with the excess methanol and sulfuric acid at high temperature to get methyl lactate. This method has a drawback that it forms large amount of salt i.e. sodium sulfate which retains almost 35 to 40% of the methyl lactate in it as reported in the same invention. Further drawback is that in order to recover methyl lactate occluded in the sludge of sodium sulfate, water is added and the methyl lactate is removed by azeotropic distillation. The isolation of lactic acid by hydrolysis of methyl lactate obtained by this process suffers major purity problem such as the crude sodium lactate used for esterification contains lot of other carboxylic acid impurities like pyruvic acid, succinic acid, acetic acid etc, which are the bye product generated in the fermentation cycle. All these carboxylic acid impurities also get converted to their respective methyl esters and contaminate methyl lactate obtained by azeotropic distillation using water and contain the trace impurities of other carboxylic acid, which contaminates lactic acid recovered by hydrolyzing methyl lactate. Further drawback of this invention is that optical purity & quality of methyl lactate used & lactic acid obtained is not reported.
Reference further may be made to the U.S. Pat. No. 2,406,648, U.S. Pat. No. 2,390,140 and U.S. Pat. No. 2,434,300 wherein a mixture of alkaline earth metal lactate preferably sodium lactate, methanol and sulfuric acid is heated to get methyl lactate. During the esterification reaction in progress, the reaction mixture pH is maintained between 0.7 to 1.4 pH values. The main drawback of the invention is, the weight ratio of methanol to lactic acid salt (on 100% basis of alkali metal salt of lactic acid) is maintained from 3 to 6, this works out the mole ratio of methanol to lactic acid as 10.5 to 21 or even up to 52.5 of mole ratio. Use of such a high amount of methanol in esterifier has clear cut drawback and it increases loading on the recovery and recycle of methanol and commercially such operations are capital intensive and requires higher running cost. Further, this method has a drawback that it forms the large amount of salt i.e. sodium sulfate as the waste product, which retains lot of methyl lactate in it. In order to recover methyl lactate occluded in the sludge of sodium sulfate, the mixture is flash distilled under reduced pressure on mineral oil. The oil recovery and recycle in such operations on commercial level become difficult and messy. Further drawback is, to purify methyl lactate; water from methyl lactate is removed by azeotropic distillation. Commercially azeotropic distillation requires large amount of energy. The optical and chemical purity of methyl lactate produced is not mentioned in this invention.
Reference also may be made to the U.S. Pat. No. 2,334,524 and U.S. Pat. No. 2,350,370; wherein a method for manufacture of lactic acid from methyl lactate hydrolysis is reported, where the methyl lactate is derived from crude lactic acid. In this invention, relatively low excess of methanol is charged in to a still with 60% to 85% of crude lactic acid and a little mineral acid, such as sulfuric acid as the catalyst. The ester i.e. methyl lactate distills of along with water and methanol in a continuous co-current manner, the distilled methyl lactate is hydrolyzed with the help of catalyst, the generated lactic acid is taken out of the hydrolysis still as the product and methanol is recycled back to the esterification still. The main drawback of this invention is that, the crude lactic acid contains lot of other carboxylic acid such as acetic acid, succinic acid and pyruvic acid as the fermentation bye products, these acids also forms corresponding esters with methanol and gets steam distilled along with methyl lactate. If the methyl lactate is not further purified by fractional distillation, the carboxylic acids generated in hydrolysis cycle of methyl lactate, gets contaminated with acetic acid, succinic acid and pyruvic acid etc., all these carboxylic acids reduces the quality of lactic acid which is not addressed in this invention.
Reference also may be made to the U.S. Pat. No. 5,210,296, wherein methods for manufacture of butyl lactate from esterification of crude ammonium lactate, excess of butanol and sulfuric acid is described. The product butyl lactate is prepared in batch mode by treating ammonium lactate with sulfuric acid and butanol and purified by fractional distillation. The patent has reported about the impurities like acetic acid and succinic acid in the crude ammonium lactate, but has not reported about the chemical purity of butyl lactate. Similarly, the process has a drawback that, it uses ammonia, which is costly base compared to lime to neutralize the lactic acid in the fermentor and does not become commercially viable option. Further to it, the process has drawback that it produces waste ammonium sulfate, which contains lot of occluded butanol in it.
Reference also may be made to the U.S. Pat. No. 6,342,626 B1, wherein a method for manufacture of methyl lactate from 73% lactic acid in two stages (at high temperature 200° C. and high pressure of 20 kg/cm2) is described. The process is carried out in two stages; first stage equilibrium conversion of lactic acid reported is about 80% and after isolating bye products of first stage, the second stage gives remaining conversion of lactic acid. The drawback of the process is that it is equilibrium-based process, involves high temperature and pressure. Further drawback is that, lactic acid being corrosive the cost of the commercial manufacturing setup will be very high. Further to it, the lactic acid is known to recemize at higher temperature (C. H. Holtan, Lactic Acid, properties and chemistry of lactic acid derivatives, Printer Oswald Schmidt KG Leipzig, 1971, page 149). The further drawback of the invention is that optical purity and the chemical purity of the methyl lactate produced is not reported.
The general method of preparation of lactic acid from crude lactic acid obtained from fermentation broth reported in the prior art is by the esterification of crude lactic acid by co-current method of reactive distillation where, the product methyl lactate, bye product water and excess methanol is taken out of reactive distillation still as a top product by maintaining the still at higher temperature. This leads to the accumulation of the acidity at the reactive distillation still and give rise to the objectionable bye-products like hydroxyl methyl furfural, 2-pentene-1-ol etc. Further as per the reported methods in the prior art, in this method of reactive distillation methyl lactate distills out along with excess methanol, the water and along with methyl esters of other carboxylic acids formed as bye product in fermentation and this mixture is further hydrolyzed without purification of methyl lactate. The purity of methyl lactate will thus govern the purity of lactic acid produced. In the prior art reported, most of the inventions do not report the impurity profile of the methyl lactate and lactic acid and its optical purity.
In the present invention, the method of preparation of methyl lactate from crude lactic acid is a continuous counter current trickle phase esterification of crude lactic acid where, the product methyl lactate is removed continuously from the reactive distillation still bottom, whereas the excess methanol, water associated with crude lactic acid and the bye product water are removed from the column top continuously. Since water is being separated from the product i.e. methyl lactate the operation mode gives higher conversion of lactic acid. Similarly this mode of operation does not allow building of excess acidity at reactive distillation still. Since the water associated with crude lactic acid and water of reaction is removed continuously by excess alcohol, the rate of formation of methyl lactate is very high; therefore there is no need to maintain the still at higher temperature to achieve complete conversion of lactic acid. Since the reaction is carried out at moderate condition so as to avoid formation of any bye-products, impurities such as hydroxymethylfurfural and 2-pentene-1-ol etc., the method of the present invention can be conveniently operated at commercial scale, Similarly, by appropriate designing, the countercurrent trickle phase mass transfer (column) approach reported in the present invention, the desired throughput as well as the complete conversion of lactic acid to methyl lactate could be achieved.
Further the method of conversion of reasonably pure methyl lactate to highly pure methyl lactate is also demonstrated. Further it is demonstrated that, the hydrolysis of highly pure methyl lactate leads to highly pure lactic acid. The method of preparation of highly pure methyl lactate from reasonably pure methyl lactate involves the use of chemical treatment of reasonably pure methyl lactate with various reagents. The reasonably pure methyl lactate contains various carboxylic acid methyl esters as the bye products formed in the fermentation cycle such as dimethyl-oxalate, dimethyl-succinate, methyl acetate, methyl pyruvate etc. Before hydrolysis of such an impure methyl lactate with water, it is first treated with sodium bicarbonate, mono-ethanolamine [141-43-5] or di-ethanolamine [11-42-2], urea or sodium-bicarbonate, mono-ethanolamine or di-ethanolamine, thiourea mixture to convert these impurities to non volatile components and the methyl lactate is isolated by distillation as highly pure S-(−)-methyl lactate which is then subjected to hydrolysis to L-(+)-lactic acid in high purity. Since, there are no additional chemicals used during hydrolysis of the highly pure S-(−)-methyl lactate, the use of highly pure water and L-(+)-lactic acid as catalyst and pre-treated carbon, results in L-(+)-lactic acid of high purity. Pre-treatment of activated carbon, where pre-treatment of activated carbon is carried out by washing activated carbon with highly pure dilute lactic acid, this helps in removal of water soluble adhered impurities on the activated carbon as a result use of such pre-treated activated carbon gives highly pure lactic acid.